564 STRUCTURE OF VERTEBRATA 



paired inferior vena cava which brings back blood from the 

 kidney (efferen.t renals), from the liver (hepatics), and from the 

 hind-limbs except when there is a renal portal system. The 

 azygos vein of Mammals is a persistent remnant of the inferior 

 cardinals, 

 (c) In Amphibia a vein known as the epigastric (anterior abdominal) 

 carries blood from the hind-limbs into the hepatic portal 

 system. This vein also receives blood from the allantoic 

 bladder, a fact which is of great theoretical importance. In 

 all higher Vertebrates in embryonic life, the blood frcnn the 

 allantois passes through the liver, and to a greater or less 

 extent into its capillaries, on its way to the heart. In 

 Reptiles the allantoic veins persist throughout life as the 

 epigastric vein or veins. In Birds and Mammals, on the 

 other hand, they atrophy completely at the close of foetal 

 life. In Birds, however, a vein is developed which connects 

 the veins coming from the posterior region with the allantoic 

 veins ; this persists when the remainder of the allantoic veins 

 atrophy, and thus in Birds as in Amphibia there is a con- 

 nection between the components of the inferior vena cava 

 and the portal system. In Mammals no such connection occurs. 

 According to many authorities, the vascular system is de- 

 veloped in the mesoderm from the hollowing out of strands 

 of cells, the outer cells forming the walls of the vessels, the 

 inner forming the constituents of the blood. The heart, with 

 the exception of its endothelial lining, is a tubular de- 

 velopment of the splanchnic mesoderm. 



Associated with the vascular system is the spleen, which 

 is an active area for the multiplication or destruction of 

 blood corpuscles, or serves as a reservoir for blood. 



The lymphatic system, developed in mesodermic spaces, 

 is a special part of the vascular system. It consists of fine 

 tubes which end blindly in the tissues and drain off fluids, 

 of larger vessels which the tubes combine to form, and 

 which open into veins. The lymph vessels contain amoe- 

 boid cells, and have associated lymphatic glands in which 

 these lymphocytes are produced. 



Respiratory system. — In Balanoglossus, Tunicates, and 

 Amphioxiis, the walls of the pharynx bear slits, between 

 which the blood is exposed in superficial blood vessels to 

 the purifying and oxygenating influence of the water. 



In Cyclostomata, Fishes, all young and some adult Am- 

 phibians, there are not only clefts on the walls of the 

 pharynx, but gills associated with these. On the large 

 surface of the feathery or plaited gills, the blood is expos^ 

 and purified. 



